Relay lenses, such as rod lenses, are often used in endoscopes to relay the image from the objective. A relay lens can include a “relay pair” that includes two rod lenses, and multiple relay lenses can be used to extend the distance from the objective to the image. That is, the length of the shaft of an endoscope can be increased by using relay lenses. Typically, an odd number of relay lenses are used in an endoscope in order to generate a right-side-up image at the eyepiece of the endoscope.
FIG. 1 shows a prior art relay lens 100 comprising rod lens doublets 101 and 102. Rod lenses 101 and 102 reimage an image formed at image plane 103 to image plane 104 along the optical path from the distal end 107 to the proximal end 108 of the relay lens 100. Image plane 103 resides between an objective lens (not shown) and rod lens 101, while image plane 104 resides between rod lens 102 and an eye piece or focusing lens (not shown), either of which can present a final image to a sensor. Alternatively, image planes 103 and 104 can be intermediate image planes which reside between relay lens 100 and additional relay lenses or other optical elements. For example, image plane 103 can be an intermediate image plane containing an image formed by an objective lens or rod lens relay and image plane 104 can contain a further intermediate image or a final image.
Conventional relay lenses contribute large amounts of astigmatism and field curvature to the image. These aberrations have traditionally been corrected in the objective optical elements. However, such correction of the aberrations solely in the objective elements leads to designs that are highly sensitive to tilt and decenter errors caused by manufacturing tolerances. For an endoscope with a large number of relays, the aberrations caused by the stack-up of these tilt and decenter errors in manufacturing tolerances can be too severe to correct solely in the objective elements. In addition, burdening the objective elements with excessive aberration correcting properties can increase the required surface curvatures and/or the optical work done by the objective elements. This can further increase tolerance sensitivity of the system.
There are other ways to compensate for the aberrations in rod lens relays. These include using the eyepiece or a combination of the eyepiece and the relays. Another solution is to design relays with fewer aberrations. U.S. Pat. Nos. 4,676,606, 4,693,568, and 7,515,355 each include information regarding aberration compensation. Many lens system designs require that any aberration correction elements be designed in conjunction with the relay lenses, which restricts the usefulness of the aberration correction elements and increases the cost of designing each lens system.
What is needed, therefore, is a way to effectively correct for aberrations in the relay lenses without over-burdening the objective elements and to reduce the tolerance sensitivity of the lens system. It is further desired to improve the compatibility of aberration correction systems with various rod lens systems.